3D Spectroscopy of Blue Compact Galaxies. Diagnostic Diagrams
Transcript of 3D Spectroscopy of Blue Compact Galaxies. Diagnostic Diagrams
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3D Spectroscopy of Blue Compact Galaxies. Diagnostic Diagrams
Ismael Martınez-Delgado1 , Guillermo Tenorio-Tagle2, Casiana Munoz-Tunon1, Alexei V.
Moiseev3
and
Luz M. Cairos4
Instituto de Astrofısica de Canarias, Vıa Lactea, E-38200 La Laguna, Tenerife, Canary
Islands, Spain;
[email protected], [email protected]
Instituto Nacional de Astrofısica, Optica y Electronica, AP 51, 72000 Puebla, Mexico;
Special Astrophysical Observatory, 369167 Nizhnij Arkhyz, Russia;
Astrophysikalisches Institut Postdam, An der Sternwarte 16, D-14482 Potsdam, Germany;
ABSTRACT
Here we present the analysis of 3D spectroscopic data of three Blue Compact
Galaxies (Mrk 324, Mrk 370, and III Zw 102). Each of the more than 22500
spectra obtained for each galaxy has been fitted by a single gaussian from which
we have inferred the velocity dispersion (σ), the peak intensity (Ipeak), and the
central wavelength (λc). The analysis shows that the σ vs Ipeak diagrams look
remarkably similar to those obtained for giant extragalactic H II regions. They
all present a supersonic narrow horizontal band that extends across all the range
of intensities and that result from the massive nuclear star-forming regions of
every galaxy. The σ vs Ipeak diagrams present also several inclined bands of
lower intensity and an even larger σ, arising from the large galactic volumes that
surround the main central emitting knots. Here we also show that the σ vs λc
and λc vs Ipeak diagrams, are powerful tools able to unveil the presence of high
and low mass stellar clusters, and thus allow for the possibility of inferring the
star formation activity of distant galaxies, even if these are not spatially resolved.
Subject headings: galaxies: spectroscopy — galaxies: individual(Mrk 370, Mrk 324,
III Zw 102) –galaxies: dwarfs — galaxies: starbursts
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1. Introduction
High resolution panoramic spectroscopy with good spatial and spectral resolution is
known to be a powerful tool for studying the kinematics of ionized nebulae as it leads
to a simultaneous mapping, at seeing limited resolution, of a particular emission line over
the whole nebula. This however can easily lead to several tens of thousands of spectra,
making it difficult to issue a detailed and/or a global interpretation of the data. An analysis
procedure that has proven to be simple and powerful results from fitting a single gaussian
to each of the resultant emission line profiles (see Munoz-Tunon 1994; Munoz-Tunon et al.
1995). A single gaussian regardless of the actual degree of asymmetry or splitting in the
line profiles. The fit is to conserve the flux of the line profile, and thus lower intensity but
broader lines would result from the most asymmetric or largely splitted line profiles. From
the resultant fits one can derive the velocity dispersion (σ), the peak intensity (Ipeak) of the
fitted lines as well as their central wavelength (λc). Such a method when applied to giant
extragalactic H II region (GHIIR) data, leads to two distinct regions in the σ vs Ipeak diagram
(see Munoz-Tunon et al. 1996): A supersonic (σ > cH II), relatively narrow horizontal band
with all possible peak intensity values, and a second region populated by lower intensity
points presenting even larger supersonic σ values, that crowd along multiple inclined bands.
For the case of GHIIRs it has been shown (see Munoz-Tunon 1994; Munoz-Tunon et al.
1996; Sabalisck et al. 1995; Fuentes-Masip et al. 2000; Telles et al. 2001, the last reference
for the case of H II Galaxies data) that the horizontal band is conformed by data with
truly gaussian profiles arising mainly from the brightest portions of the nebula, from the
small regions that enclose groups or clusters of stars. The inclined lower intensity and
highly supersonic bands on the other hand, emanate from multiple photoionized shells that
surround the star-forming centers and are likely to result from the stellar mechanical energy
impact into the ISM, and thus from single gaussian fits to highly asymmetric or strongly
splitted lines. The analysis of the σ vs Ipeak diagram for GHIIRs in nearby galaxies (closer
than 1 Mpc) was then proposed as an excellent tool to determine their degree of evolution on
kinematic bases (Munoz-Tunon et al. 1996). Here we extend the analysis to Blue Compact
Galaxies (BCGs) at even larger distances and show that they experience a similar dynamical
evolution (ie. their σ vs Ipeak diagrams look very much like those of GHIIRs). We further
extend our analysis by looking also at the σ vs λc and λc vs Ipeak diagrams, and show them
as ideal tools to distinctly unveil the presence of high and low mass stellar clusters.
Here section 2 provides a short description of the observations. Section 3 contains a
detailed analysis of the data by means of the diagnostic diagrams here proposed and section
4 gives a summary and a discussion of our findings.
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2. The observations
We use the 6m Big Telescope Alta-azimuthal (BTA) of the Special Astrophysical Ob-
servatory (SAO) in Nizhnij Arkhyz (Russia), equipped with the multi-mode focal reducer
SCORPIO1 in the scanning Interferometer Fabry-Perot (IFP) observational mode (see Afanasiev & Moiseev
2005), for the observations of three BCGs: Mrk 370, III Zw 102, and Mrk 324. SCORPIO
is provided with a EEV 42-40 CCD array of 2048 × 2048 pixels with an instrumental scale
of 0.18 ′′ pixel−1. A set of interference filters centered at the Hα line, covering an interval of
systemic velocities that range from −200 to 10000 km s−1, is used at the IFP observational
mode. The IFP offers two configuration depending on the interference order, in our case at
the Hα line, the IFP260 (at the interference order 235) and the IFP500 (at the interference
order 501). Our observation runs were performed using the IFP500 mode, that offers a better
spectral resolution in the Ha emission line.
The output from the IFP results on a 3D data cube with x and y being the spatial
plane and z the wavelength sampling or ethalon step, that in the case of IFP500 mode, it
translates into 32-40 channels. The original data cube dimensions were (522,522,36) although
for our analysis we reduced the size, matching the objects size, to (150,150,36). The Free
Spectral Range (FSR) was 13 A, enough to sample the Hα line, that was scanned with a
spectral sampling of 0.7 A ≈ 32 km s−1. A binning of 4×4 pixels was applied in the spatial
directions in order to reduce the time exposure and improve the signal to noise of the final
data cube, resulting in a final spatial scale of 0.7 ′′ pixel−1. The instrumental width taken
from a fitting of the calibration lamp data cube was σinst = 20 ± 3 km s−1. Table 1 lists
a log of the observations together with some data about the galaxies: Column 1 gives the
name of the object; columns 2 and 3 show the coordinates (from NED2); columns 4 shows
the linear scale for each galaxy; column 5 lists the total time exposure of the observations;
column 6 gives the seeing conditions; column 7 shows the systemic velocity of each galaxy
and column 8 gives the distance in Mpc. Each of our SCORPIO trimmed data cubes contains
a total of 22500 spectra from an area of ∼3 arcmin2. The bulk of the data thus requires of
automatic techniques of analysis to extract the full power of 3D spectroscopy. For each of the
three galaxies, their Hα emission lines in every pixel were fitted by single gaussians, to then
obtain their Ipeak, σ, and λc, as described in Munoz-Tunon et al. (1995, 1996). The velocity
dispersion is given in km s−1 and is corrected for the instrumental and thermal broadening
(σ2 = σ2
obs − σ2
inst − σ2
th). We then used MoisSoft, the software package designed for the
1http://www.sao.ru/hq/lsfvo/devices/scorpio/scorpio.html
2NASA/IPAC Extragalactic Database is operated by the Jet Propulsion Laboratory, California Institute
of Technology, under contract with the National Aeronautics and Space Administration(NASA).
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manipulation of SCORPIO-IFP spectral line data cubes. For a detailed explanation about
the data reduction procedure, see Moiseev (2002).
3. Three Dimensional Spectroscopy of BCDs
3.1. Mrk 324
Mrk 324 is a galaxy classified as a Blue Compact Dwarf (BCD, see Thuan & Martin
1981) and is included as a Nucleated BCD in the morphological classification of Cairos
et al. (2001), as it is basically powered by a luminous central knot. The star formation
history of the galaxy fits well with an instantaneous star formation law with a Salpeter IMF
(α = 2.35) and a range of masses between 1≤ M/M⊙ ≤100, in which a central knot of
4.4 Myr and 1.4×105 M⊙ in stars dominates 95% of the emission flux of the galaxy (see
Martınez-Delgado et al. 2006).
Figure 1 (upper row of panels) shows the results from the single gaussian fit to the full
Hα SCORPIO data set in the σ vs Ipeak, σ vs λc, and λc vs Ipeak diagrams for Mrk 324. The
last frame, upper row, shows the Hα image of the nucleated galaxy (see Cairos et al. 2001)
obtained at the NOT 2.52m telescope of the Roque de Los Muchachos Observatory (ORM)
in La Palma (Spain). The resemblance of the σ vs Ipeak diagram with those of GHIIRs is
remarkable. It presents a horizontal band that limits the values of σ to supersonic values (σ
≥ 20 km s−1) and a bunch of low intensity points at even higher σ values, that delineate a
triangular structure above the horizontal band. If one selects from the σ vs Ipeak diagram
the highest intensity data points, those belonging to the horizontal band (see second row of
panels in Figure 1), these in the σ vs λc diagram appear clumped at a given λc (∼ 6598.2
A) what implies that they all arise from a very similar location in the galaxy, that they all
belong in principle to a single entity. Note however, that this is not exactly coincident with
the rest wavelength of the galaxy (marked in the λc plots with a red line, see Figure 1).
The selected points in the λc vs Ipeak diagram defined also a pointed structure towards high
intensity values. These points in the emission-line peak intensity map (last frame) display
their location in the galaxy, which indeed coincides with the major burst of star formation
in Mrk 324. A major burst with an estimated mass in stars ∼ 4 × 105 M⊙ and an age of
about 4.4 Myr (see Martınez-Delgado 2005; Martınez-Delgado et al. 2006) that causes an H
II region with a radius of ∼ 250 pc. On the other hand, the data belonging to the inclined
bands with a higher velocity dispersion and a lower intensity in the σ vs Ipeak diagram (see
third row of panels in Figure 1) appear well scattered in λc and arise from a large-scale (600
pc in thickness) expanding rim that surrounds the central emitting knot, as shown in the last
panel of the third row. The inserts in the last column of Figure 1 display the Hα line and its
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single gaussian fit at various locations in the galaxy. As in GHIIRs, the data corresponding
to the horizontal band in the σ vs Ipeak diagram leads to the highest intensity true gaussian
profiles, while the profiles fitted to lower intensity and larger σ data points, arising from the
outer expanding rim, are fits to asymmetric and/or double peak line profiles. Figure 1 shows
the quality of typical line profiles from each of the regions, displaying their intensity, central
wavelength, and σ values.
3.2. Mrk 370
This BCD galaxy belongs to the Chained morphological type (see Cairos et al. 2001),
in which the star formation takes place along a privileged direction. It follows the same
star formation law as Mrk 324, with 25 young and massive exciting clusters hosted over
an underlying stellar component with ages larger than 5 Gyr, (see Cairos et al. 2002). The
clusters have an average age of 6.6 Myr with a dispersion of 0.9 Myr, suggesting a coeval
and global star formation event. For the ones located in the inner nuclear region, the stellar
mass content reaches up to several 105 M⊙, (see Martınez-Delgado et al. 2006).
Figure 2 displays similar diagnostic diagrams for Mrk 370. The last frame in the upper
row of panels shows the Hα image of the chained galaxy (see Cairos et al. 2001) obtained at
the CAHA 2.2m telescope of the Centro Astronomico Hispano Aleman (CAHA), from Calar
Alto Observatory (Spain). The ionized gas in Mrk 370 (see the Hα image in Figure 2) presents
much more structure than Mrk 324. However, its σ vs Ipeak diagram looks almost identical
to that shown in Figure 1 for Mrk 324. The only exception is perhaps the low intensity data
points with values of σ below the supersonic lower limit imposed by the horizontal band (≤
21 km s−1, see first panel in Figure 2). The structure of the photoionized gas in Mrk 370
begins to be disentangled when one selects the data from the horizontal band in the σ vs
Ipeak diagram (see Figure 2, second row of panels). Such a data set appears clumped at three
different λc values in the σ vs λc diagram. The brightest one sits at the rest wavelength
of the galaxy (6580.1A). There is a second one at ∼6579.4A and a third one between these
two at ∼6579.8A. Two of them are much more apparent in the λc vs Ipeak diagram where
they clearly end up as pointed structures with different intensity (factors of 2) and different
central wavelengths. The emission-line peak intensity map tracing the selected points (last
frame in the second row) shows two major bursts of stellar formation and two adjacent much
less intense, associated to the massive central stellar clusters with masses that range between
(8−50)×104 M⊙ (see Martınez-Delgado 2005; Martınez-Delgado et al. 2006), also resolved
by Cairos et al. (2002) using the INTEGRAL spectrograph with a single gaussian fit to the
emission lines. All of these, although near the center of the galaxy, are kinematically resolved
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as well detached structures. This multiplicity of structures would have been missed if one
had only use the σ vs Ipeak diagram as a diagnostic for this galaxy (see first panel in Figure 2),
as the brightest points present almost the same supersonic gaussian profiles. These massive
regions also match the area of the galaxy that shows the highest ionization values (see the
Hα emission-line and the [OIII]/Hβ maps in Cairos et al. 2002).
The third row of panels in Figure 2 displays the data from the inclined bands with low
intensity and large σ values in the σ vs Ipeak diagram. This data set, as the corresponding
one in Mrk 324, presents a large range of values of λc, around the systemic velocity of the
galaxy. The spread of the data is also noticeable in the λc vs Ipeak plane. The bulk of the
data arises from a large galactic volume shown in the emission-line peak intensity map as an
expanding rim of gas around the main centers of star formation all over the galaxy. This is
most intense around the nuclear starburst zone and although less intense, it is still noticeable
around smaller bursts of stellar formation some 2 kpc away from the nuclear region.
The final row of panels show the low intensity and low velocity dispersion data from the
σ vs Ipeak diagram (see Figure 2), which appears as vertical well detached low σ structures
in the σ vs λc plane. These structures, given their low values of σ, detach well from the bulk
of the data, most of which presents supersonic values, and due to their largely different λc
values they also appear as well detached entities along the λ scale. Note however that given
their low intensity values, they are much less apparent in the λc vs Ipeak diagram. These
data points correspond in the emission-line peak intensity map (last panel in the 4th row
of Figure 2) to multiple knots of emitting gas, or H II regions excited by low mass stellar
groups (with masses ≤ 104 M⊙, see Martınez-Delgado 2005; Martınez-Delgado et al. 2006)
at large distances from the centre of the galaxy. The line profiles arising from these regions
are also gaussian profiles although with a very low intensity when compared to those arising
from the central starburst regions (see inserts in the last frame of Figure 2).
3.3. III Zw 102
This galaxy is believed to be the result of an interaction event (see Vorontsov-Velyaminov
1959; Vorontsov-Vel’Yaminov 1977) and has been considered by some authors to belongs to
the polar-ring galaxy type (Whitmore et al. 1990). It is morphologycally considered as an
Extended Blue Compact Galaxy (Cairos et al. 2001), in which 65 star-forming regions were
identified spreaded over the whole main body of the galaxy, including the arms. The age of
the knots and their low age dispersion (6.1 ± 0.6 Myr) suggest a coeval starburst event (see
Martınez-Delgado et al. 2006).
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Figure 3 displays the results for III Zw 102. The last frame in the upper row of panels,
shows the Hα image of the extended galaxy (see Cairos et al. 2001) obtained at the NOT
2.52m telescope of the Roque de Los Muchachos Observatory (ORM) in La Palma (Spain).
The data for III Zw 102 confirms the power of all of the diagnostic diagrams here used.
The galaxy has a plethora of small H II regions spread along two arms as well as several
massive centers of stellar formation in the densest nuclear region with masses between ∼
105 − 106 M⊙, (see Martınez-Delgado 2005; Martınez-Delgado et al. 2006). All of these, as
well as the large galactic volumes undergoing a supersonic expansion leave their signature
in the diagnostic diagrams. In this way, the horizontal band with supersonic σ values at all
possible intensities noticeable in the σ vs Ipeak diagram (see Figure 3, 2nd row of panels) is
spread over a large range of λc values in the σ vs λc diagram and into at least four pointed
structures (at λc = 6596.7A, 6597.25A, 6598A, and 6600A) of different intensities in the λc
vs Ipeak diagram. All of these in the emission-line peak intesity map are well resolved as
giant structures within the galactic nuclear region. On the other hand, the larger σ points
at all peak intensities in the σ vs Ipeak diagram arise from gas expanding around the nuclear
zone (see third row of panels in Figure 3). Among this data set is the inclined structure
between Ipeak=1100-1400 which is produced by gas between the four major centers of stellar
formation in the nuclear region. The broad line structure appears at the center of the galaxy
with a λc very similar to the systemic velocity of III Zw 102.
Finally, the lowest σ and lowest intensity data points in the σ vs Ipeak diagram (last row
of panels in Figure 3) appear, as in the case of Mrk370, as a set of well detached vertical
stalactites in the σ vs λc diagram, hanging from the bulk of the data at very distinct λc values.
The latter as shown in the last panel are produced by the numerous small H II regions located
along the galactic arms. As shown by the inserts in the last column of panels, the quality of
the gaussian fits is totally different in the three regions. The nuclear zone shows supersonic
perfect gaussians while the surrounding gas presents much broader and asymmetric lines.
Single gaussians with small σ values fit well the data from small centers of star formation.
4. Summary
Tridimensional spectroscopy with good spatial and spectral resolution, sampling a par-
ticular emission line over the whole area covered by an emitting nebula, is known to be the
most suitable observational technique for analyzing the global kinematics of GHIIRs. In
particular if a single gaussian fit is carried out over the emission lines in every pixel and from
those, one infers the velocity dispersion, the central wavelength, and the peak intensity of
the lines. The resultant σ vs Ipeak diagram (Munoz-Tunon et al. 1996) has been shown to
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be an excellent diagnostic diagram to separate the main broadening mechanisms affecting
the emission lines, e.g., those that lead to shells and loops generated by the violent action
from massive stars (see Dyson 1979, Roy et al. 1986) which finally lead to cloud dispersal,
and the one(s) affecting the regions of massive star formation (either gravity; see Terlevich
& Melnick 1981, Tenorio-Tagle et al. 1993, or turbulence; see Chu & Kennicutt 1994). The
sequences shown by the various GHIIRs so far analyzed, led the authors also to propose the
σ-Ipeak plot as a tool able to trace the evolutionary status of a GHIIR, from its formation to
the total dispersal of the ionized gas.
Here we have confirmed that the data from the inclined supersonic bands in the σ vs
Ipeak diagrams are caused by the mechanical energy deposited by massive stellar sources.
These, depending on the age of the stellar clusters and on the density of the surrounding
gas may affect small or large galactic volumes. However in all cases, the interaction leads to
very asymmetric line profiles, evident in the σ vs Ipeak diagram for their highly supersonic σ
values.
Here we have extended the analysis to Blue Compact Galaxies. We have selected sources
with a single nuclear starburst (as Mrk 324) as well as galaxies with multiple stellar bursts
both within their nuclei as well as at large galactocentric distances. In all cases the σ vs Ipeak
diagram resembles those from GHIIRs in which the data define a supersonic σ (larger than
cH II ≈ 10 km s−1) horizontal band with all possible intensity values. Note that the value of
the limiting σ is different in every case. For III Zw 102 the horizontal band peaks at ∼ 33
km s−1, while for Mrk 324 and Mrk 370 it occurs at 24 and 25 km s−1, respectively.
The σ horizontal band, for the case of multiple massive nuclear bursts of stellar forma-
tion, has been shown to split into different bands in the λc vs Ipeak diagram. In this new
diagram, the largest nuclear regions of massive star formation become evident due to the
good spectral resolution. And thus even with a poor spatial resolution, or when dealing with
further away sources, the sampling of the velocity field, with our 3D spectrographs, would
reveal the exciting sources. Similarly, the σ vs λc new diagram picks up the small bursts of
stellar formation by tracing their lower intensity and slower expansion of their immediate
surroundings.
The stalactites detected in the σ vs λc diagram in the case of III Zw 102 and Mrk 370
correspond to the ionized knots located in the outskirts of the galaxies and their spread in
σ result from the H II regions expansion into a low density medium (see Franco et al. 1990).
On the whole, the new diagnostic diagrams here presented provide the possibility of
inferring from 3D spectroscopy, the magnitude of the star formation activity of distant
galaxies, even if these are not spatially resolved.
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IMD acknowledges the FPI grant (FP-2001-2506) of the Spanish Government through
the collaboration of the project AYA2004-08260-C03-01 (ESTALLIDOS,
http://www.iac.es/project/GEFE/estallidos). AVM acknowledges the the Russian Founda-
tion for Basic Research (project 05-02-16454). This work is partly based on observations
carried out at the 6m telescope of the Special Astrophysical Observatory of the Russian
Academy of Sciences, operated under the financial support of the Science Department of
Russia (registration number 01-43), and it has been partly supported by the ESTALLIDOS
Project (AYA2004-08260-C03-01) and the grant AYA 2004-02703 from the Spanish Ministe-
rio de Educacion y Ciencia.
– 10 –
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This preprint was prepared with the AAS LATEX macros v5.2.
–12
–
σ=46.0 km/s
σ=33.9 km/s
σ=23.9 km/s
σ=24.4 km/s
∆ x(")
∆y(
")
v(km/s)
Fig. 1.— The figure shows in the first row of panels the velocity dispersion σ vs peak intensity (the σ was corrected for the instrumental and
thermal broadening), for all the gaussian fits to the individual spectra of Mrk 324. The second and third panels display the corresponding σ
vs central lambda (λc, indicating also the velocity range in km s−1) and λc vs peak intensity, respectively. The solid line in these two panels
indicates the rest velocity of the galaxy. The last panel shows the Hα image of the galaxy and indicates its scale in kpc. The second and
following rows of panels highlight in colour the various sets of data points selected in the σ vs peak intensity diagram and their location in
the σ vs λc and λc vs peak intensity planes. The last panels in every row, with a peak intensity scale, display the locations in the galaxy
that produce the selected data points. Inserts in these panels correspond to typical gaussian fits (solid red lines) to the data arising from
different regions (axis correspond to intensity, in counts, vs the wavelength in A). The fitted σ values are indicated in every frame.
–13
–
Fig. 2.— Mrk 370. The same as Figure 1 for Mrk 370.
–14
–
Fig. 3.— III Zw 102. The same as Figure 1 for III Zw 102.
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Table 1. Log of the Observations
Galaxy RA(2000) DEC(2000) Linear Scale(pc/′′) t(s) PSF(′′) VHI(km s−1) D(Mpc)
(1) (2) (3) (4) (5) (6) (7) (8)
Mrk370 02h40m29.s0 +19◦17′50′′ 52 4680 1.7 790† 10.85
IIIZw102 23h20m30.s1 +17◦13′32′′ 110 6480 1.8 1626† 22.71
Mrk324 23h26m32.s8 +18◦15′59′′ 108 4320 1.6 1600† 22.43
†Systemic velocity, from H I observations, corrected to the Local Group velocity centroid taken
from Thuan & Martin (1981).